Impulse heat sealing apparatus for heat sealing thermoplastic materials such as plastic bags are known in the art. The heat sealer, referred to as an impulse sealer, uses an electrically conductive high resistance wire heated generally by a short burst of electrical current. While heated, the wire is pressed against or through the materials to be sealed.
High resistance wire is relatively flexible so it can be used for making either linear or non-linear heat seals. In the context of the present invention, "non-linear" is understood to mean a seal or seam which is curved or has a contour or shape other than straight across the flat width of the bag.
Various arrangements are available for mounting and supporting the resistance wire in operation. For example, in one typical arrangement the wire is held in tension by spring biased members attached only to the ends of the wire. This keeps the wire straight and the spring bias serves to accommodate the elongation and contraction of the resistance wire during its heat up and cool down cycles. Since the wire is held in tension, this arrangement is limited to making straight seals and sealing is accomplished by passing the wire completely through one or more plies of thermoplastic materials.
In another arrangement, the wire is rested on one face of an elongated support bar usually a metal bar which functions both as a support for the wire and as a heat sink. With this arrangement the wire must be insulated from the bar and is pressed against, but not necessarily through, the materials to be heat sealed. Either straight or non-linear seals can be made with this arrangement. However, for non-linear seals, the wire must be bent to the desired seal configuration and then anchored along its length to the support bar to maintain the desired shape. For example, in one arrangement the resistance wire is laid in the desired shape onto a support bar and secured in place with an adhesive tape. The adhesive tape conventionally used is one having a backing composed of an insulating fabric such as a fabric woven from glass fibers and impregnated or coated with tetrafluoroethylene (TFE). This electrically insulates the wire and prevents the heated wire from sticking to the heat sealed plastic. To insure that essentially the same shape of seal is made when either the tape or wire is replaced, the support bar may have a groove of the desired seal shape for receiving a wire laid into this groove.
Anchoring the wire with an adhesive tape, with or without the added support of a groove in the support bar, requires some skill. Allowance must be made to accommodate the expansion and contraction of the wire yet it must be so secured that it maintains the desired configuration even after repeated cycles of expansion and contraction. Movement of the wire from the desired shape of seal configuration often is not detected until after a seal is made. When this occurs, the heat sealing operation must stop while the adhesive tape is removed and the wire is realigned or replaced and then is retaped into position.
Another anchoring scheme is disclosed in U.S. Pat. No. 3,334,005. Here, spikes are welded to the sealing wire at spaced intervals along the length of the wire. The spikes are then anchored in holes drilled into the support bar. While this insures the wire is fixed in the desired seal shape, the spikes provide heat sinks which interfere with the uniform heating of the wire so there may be cold spots at intervals along the wire. To insure proper heat sealing with such an arrangement some accommodation must be made to compensate for these cold spots. For example, the '005 Patent discloses use of separate heating elements to heat the spikes and prevent such cold spots along the wire.
The present invention provides means for mounting a flexible heat sealing element to produce curved or other non-linear heat seals. The mounting means of the present invention holds the heat sealing element in the desired shape or contour and all non-linear seals are maintained to the desired contour regardless of the number of seals made. Moreover, in the present invention, the flexible heat sealing element is free to expand and contract longitudinally over its entire length without altering the seal contour. Also, the mounting means of the present invention makes it possible to remove and replace flexible heat sealing elements quickly and easily. No special skills are required to insure that the replacement heat sealing element makes the same shape non-linear seal as the one replaced.
Accordingly, one object of the present invention is to provide an impulse heat sealer for making either linear or non-linear heat seals.
Another object of the present invention is to provide a heat sealer for making either linear or non-linear heat seals which allows an electrically heated sealing element to freely expand or contract without altering the shape of the heat seal.
A further object of the present invention is to provide an impulse heat sealer for making non-linear heat seals wherein changing a flexible heat sealing element is accomplished with a minimum of effort and skill and without altering the shape of the heat seal.